Efficient Markets, Part 5: The Case for Index Funds
If you can’t beat the market — and the evidence says most people can’t — maybe the right move is to own the whole thing.
In 2013, Warren Buffett wrote something in his annual letter to Berkshire Hathaway shareholders that caught people off guard.
He was describing what should happen to his wife’s money after he dies. His advice was specific: put 90% of it in a very low-cost S&P 500 index fund.
Pause on that for a second.
Warren Buffett is the most famous active stock investor in recorded history. He has picked individual stocks for sixty years. His compound annual return over that period is roughly 20%. He is, by any reasonable measure, the greatest stock-picker who has ever lived.
And he told his wife to buy an index fund.
This post explains why. It follows directly from everything we’ve covered in Parts 1 through 4. If you’ve been reading along, you’ve already built the foundation. Now we get to the conclusion.
A Quick Recap of Where We’ve Been
The last four posts built a single argument in steps.
Part 1 introduced the random walk hypothesis — the idea that stock prices move in response to new, unpredictable information, not in patterns that can be predicted from price history.
Part 2 showed that technical analysis — predicting prices from charts and patterns — fails to outperform the market consistently once you account for costs.
Part 3 showed that fundamental analysis — modeling a company’s finances to find undervalued stocks — faces the same challenge. Most professional analysts already do this. Their work is already in the price.
Part 4 laid out the full Efficient Market Hypothesis in three forms, and reviewed the evidence. The Weak Form (prices reflect past price data) and the Semi-Strong Form (prices reflect all public information) are both well-supported. The implication is that most approaches to beating the market, for most investors, are unlikely to work over the long run.
So: what do you do with that?
What Is an Index Fund?
Let’s start with the basics, because this term gets thrown around so often that its meaning sometimes blurs.
A stock market index is a list of companies, selected and maintained according to specific rules. The most famous is the S&P 500 — five hundred large American companies, chosen by a committee at S&P Global to represent the broad U.S. economy. When people say “the market was up 1.3% today,” they usually mean the S&P 500 was up 1.3%.
An index fund is an investment fund that simply holds all the stocks in an index, in proportion to their size. If Apple makes up 7% of the S&P 500 by market value, the fund holds Apple at 7% of its assets. If a small regional bank makes up 0.02%, the fund holds that too.
When you buy a share of an S&P 500 index fund, you own a tiny sliver of all five hundred companies at once. You don’t pick which ones. You don’t hire an analyst to find winners. You own the whole thing.
This is called passive investing — because the fund isn’t actively deciding what to buy or sell. It just tracks the index. Its return equals the index return, minus a very small fee.
The alternative — having a manager actively decide which stocks to buy and sell in search of higher returns — is called active investing. Most of this series has been about why that’s harder than it sounds.
The Professional Track Record
Here’s the problem that launched the index fund movement.
Most professional active fund managers underperform their benchmark index — measured over the long run, after fees.
Not “slightly underperform.” Not “miss by a little in one bad decade.” Consistently, reliably, in study after study.
The SPIVA Scorecard — published twice a year by S&P Global — tracks how active fund managers perform against their relevant benchmark. The numbers are blunt. Over any 15-year period, more than 85% of actively managed large-cap U.S. stock funds underperform the S&P 500 after fees.
That figure deserves to sit with you for a moment.
These are professionals. People with decades of experience, enormous research budgets, proprietary data, and whole teams of analysts. More than eight out of ten of them, measured over long enough periods, do not beat a simple index.
And here’s what makes it worse: past strong performance does not predict future performance. The fund that was top-quartile last decade tends to be average or below-average in the next decade. So even if you could identify last decade’s winners, you couldn’t use that to pick next decade’s winners. The research on this point is decisive.
Burton Malkiel, whose book this series draws on, frames it this way: if you can’t beat the market, and you can’t reliably identify who will beat the market, then paying for active management is pure cost with no expected benefit.
The Hidden Killer: What Fees Actually Cost You
Now here’s the math. This is where the index fund case gets really concrete.
When you invest in an actively managed fund, you pay an expense ratio — an annual fee, expressed as a percentage of your account balance, that covers the fund’s operating costs. Analyst salaries. Portfolio manager compensation. Trading costs. Office overhead.
The fee doesn’t appear as a bill in your mailbox. It’s deducted automatically from the fund’s assets. But it comes out of your return either way.
A typical actively managed large-cap fund charges somewhere between 0.5% and 1.0% per year.
A typical S&P 500 index fund — from Vanguard, Fidelity, or Schwab — charges between 0.03% and 0.10% per year.
That difference seems trivial. Over a lifetime of investing, it’s anything but.
Let’s run the numbers on a $10,000 investment, assuming a 7% annual return from the underlying market:
| Annual Fee | Value After 30 Years | |
|---|---|---|
| Index fund | 0.05% | ~$75,000 |
| Active fund (low-cost) | 0.75% | ~$62,000 |
| Active fund (typical) | 1.00% | ~$57,000 |
The index fund investor ends up with roughly $13,000 to $18,000 more — from the same underlying market. Every dollar of that difference went to the fund company, not to you.
And that’s assuming the active fund matches the index before fees. Most don’t. So the actual gap is usually wider.
This is the power of compounding in reverse. The same math that grows your savings also grows the cost of fees, year after year. A 1% annual drag doesn’t cost you 1% of your ending wealth. Over thirty years, it costs you roughly 20%.
What You Own When You Own the Market
There’s another argument for index funds that doesn’t get enough attention, and it’s not about costs.
When you buy an index fund, you’re not buying a financial abstraction. You’re buying ownership stakes in hundreds of real businesses — the companies that build the phones in our pockets, staff the hospitals in our cities, grow the food on our tables, and write the software that runs the global economy.
Over the long run, stock returns reflect the growth of corporate earnings. The S&P 500 has returned approximately 10% per year on average over the past century (about 7% after inflation). That return comes from the underlying economic output of American business — not from anyone’s analytical genius.
Diversification — owning many investments so no single failure can destroy you — comes built-in. An S&P 500 index fund holds five hundred companies across every major industry. If one goes bankrupt, its weight in your portfolio was 0.2% or less. You barely feel it.
Think of it this way: picking individual stocks is like trying to find the winning horse in a race. Owning an index fund is like owning a small piece of all the horses. You don’t win as spectacularly — but you can’t be wiped out by a single stumble.
Malkiel’s Conclusion
Burton Malkiel spent decades studying the performance data on active vs. passive investing. His conclusion — first published in A Random Walk Down Wall Street in 1973, updated through thirteen editions — is straightforward:
For most investors, most of the time, a low-cost diversified index fund is the best investment they can make.
Not because picking stocks is impossible. Not because no one ever beats the market. But because the average investor, over the long run, will almost certainly do better in an index fund than by trying to select individual stocks or active managers.
The reason flows directly from the logic of this series. If markets are mostly efficient — if prices mostly reflect available information — then there is no reliable, systematic way for most investors to extract consistent advantage from that information. And if you can’t extract an advantage, then you’re paying management fees for nothing.
Buffett’s Endorsement — and What It Actually Means
Back to that 2013 Berkshire Hathaway letter.
Buffett’s reasoning was plain: “The goal of the non-professional should not be to pick winners — neither he nor his ‘helpers’ can do that — but should rather be to own a cross-section of businesses that in aggregate are bound to do well. A low-cost S&P 500 index fund will achieve this goal.”
He later went further, describing a famous decade-long bet he made against a group of hedge funds. He wagered that a simple S&P 500 index fund would outperform a basket of professionally managed hedge funds over ten years. The index fund won by a wide margin.
Buffett’s endorsement of index funds is often misunderstood. He’s not saying stock-picking is useless. He is, after all, a stock-picker. He’s saying that for most people — people who don’t have his skills, his patience, his analytical framework, or his forty thousand hours of experience — the index fund is the honest, high-probability path.
The skill required to beat the market is real. It is rare. It cannot be bought by picking the right fund manager. And it cannot be developed without years of genuine study and practice.
For most investors, the index fund isn’t plan B. It’s the right plan.
What This Means for You
A few practical takeaways from everything we’ve covered.
Costs are not small. Before investing in any fund, read the expense ratio. The return you earn minus the fee you pay equals what you actually get. Two funds with identical underlying assets can produce very different long-run outcomes based on fees alone.
Diversification is not optional. Owning five stocks is not a portfolio. Owning five hundred stocks through an index fund is. The difference in risk is enormous.
Past fund performance tells you almost nothing. The SPIVA data is clear: last decade’s winner is not reliably this decade’s winner. Choosing an active fund based on its recent hot streak is likely to disappoint.
The market return is not a consolation prize. Most investors, most of the time, don’t earn the market return — because they trade too much, buy high and sell low, pay too many fees, or bail out at the wrong moment. Simply earning the market return, consistently, over many decades, is a genuine achievement. An index fund makes it possible.
A Note to Luca and Lili
I want to be honest with you here, because there’s a real tension in this series.
Everything we cover in The Rules, in Intrinsic Value, in Growth Investing — all of it is built on the idea that careful analysis can identify undervalued companies. This series has spent five posts making the case that it’s hard. Very hard. And that for most investors, the evidence points to the index fund.
Both things are true, and they are not as contradictory as they might look.
If you are just starting out — if you don’t yet have the skills, the time, or the experience to do deep fundamental analysis — a low-cost S&P 500 index fund is genuinely the right answer. Buy it. Reinvest the dividends. Leave it alone for decades. You will beat most professional active managers and virtually all unsophisticated individual investors. That’s not a consolation prize. That’s a real outcome.
As you develop your skills — working through the models, studying financial statements, building intuition for what makes a business durable — you may find situations where the market is genuinely mispricing a company. Those opportunities exist. They are smaller and rarer than most investors imagine, but they are real.
Your grandfather built his analytical system not because he thinks the market is easily beaten. He built it because he believes the discipline of the search — the honest effort to understand businesses at a deep level — produces better decisions than the alternative, even when the market is mostly right.
Start with the index fund. Build toward something more. Never confuse the two.
— Papa
The One-Sentence Summary
The case for index funds flows directly from market efficiency: because prices rapidly reflect available information, most active fund managers cannot consistently beat their benchmark after fees, and for most investors, a low-cost diversified index fund — which captures the long-run growth of the market at minimum cost — will outperform most alternatives over the long run.
The argument in this post arrives at the same practical conclusion from an academic angle — market efficiency makes systematic outperformance very hard. For the same conclusion reached through pure arithmetic, see The Bogle Method, Part 1: Stop Trying to Beat the Market — Jack Bogle’s own case for why the lowest-cost path captures the most return.
Next: Efficient Markets, Part 6 — Where Active Investing Still Finds the Edge. We have spent five posts building the case for market efficiency. The argument is strong. But it is not complete. There are situations where active investors with genuine skill, disciplined process, and long time horizons can find real advantages over a market index — and those situations are worth understanding carefully. In Part 6, we look at exactly where those edges exist, and how to tell the difference between a real edge and wishful thinking.
— Jim